If your run did reach Toxic Flame Kingdom, when reloading to a new file and entering the kingdom again, there is a high chance it might crash.

My work-around:

• After a post-Toxic-Flame run, load a pre-Toxic-Flame file (I just use a Cap Kingdom, 0 moon file)
• Wait until the game saves
• Close and reopen the game
• Load a new file and start a run